Through evolution, many human civilizations have learned they have a better chance of survival if they form communities of members having different talents than if they go it alone.
Many foodborne pathogens have developed similar group survival strategies through "biofilms" - communities of multiple bacterial species that typically are more resistant to antimicrobial treatments than are free-floating organisms.
It's the activity of antimicrobials on the specific pathogen, Listeria monocytogenes (Lm), within these multi-species biofilms that Dr. Rolf Joerger, an associate professor with the University of Delaware's Department of Animal and Food Sciences, is addressing through a two-year study. Titled "Evaluation of the efficacy of antimicrobial agents to prevent the transfer of Listeria monocytogenes from existing biofilms to produce or processing surfaces
," it is expected to
yield data the produce industry and regulators can use to develop further packinghouse treatment guidelines.
"In general, it's fairly easy to kill bacteria that are just swimming around in a liquid - such as wash water," he said. "If there are individual cells there, then you put chlorine or another antimicrobial in and you can kill them -o
nce the bacteria attach to the surface, it becomes more difficult for the antimicrobial to kill them because a lot of bacteria form certain protective layers over time."
The project also
seeks to examine whether Lm dislodged from biofilms is more resistant to antimicrobials than are free-floating Lm cells. In addition, Joerger plans to study how to prevent formation of new Lm-containing biofilms on produce and uncontaminated equipment surfaces.
g the first year, Joerger examined the biofilm-forming capability of 23 different Lm strains grown on media as well as in lettuce wash water and filtered lettuce extract. From there, he narrowed it to three Lm strains and subjected the biofilms to various concentrations of hypochlorite to determine the time it took for
After talking to colleagues at the CPS Produce Research Symposium this summer, Joerger said he has modified his research to look at more complex biofilms composed of multiple bacterial species, including Lm. But Listeria monocytogenes still remains the focus.
"In the real world, there are probably hundreds of different bacteria that can form biofilms on produce harvesting and processing surfaces, and Listeria will mix with those and will be in a biofilm composed of multiple bacterial species in that environment," he said. "It's possible that Listeria is more protected in those biofilms than just by itself."
The challenge now is to create multi-species biofilms representative of those found on packinghouse surfaces.
Joerger, along with two graduate students, is in the process of isolating bacteria from packinghouse samples and narrowing that to four or five different species.
They then will subject those multi-species biofilms to various antimicrobials to determine how long it takes for Lm inactivation. He admitted it's a challenge using concentrated enough antimicrobial solutions and long enough contact times to deactivate the biofilm-protected Lm without damaging produce.
Eventually, Joerger said, he plans to look at how organic matter may influence antimicrobial activity on Lm biofilms. But he wanted to work initially with a limited number of variables before introducing another one.
"Organic matter is one of the biggest problems," he said. "We're trying to simulate those kinds of issues as much as possible by adding organic matter and using actual wash water and making extracts from produce. But that always complicates things."